(1) Field of the Invention
The present invention relates generally to a circuit for forming a pulse train output suitable for use with, e.g., a laser interferometer.
(2) Background of the art
In general, to derive a physical quantity using pulses output from a predetermined sensor, a product of an accumulated number of pulses and a reference unit quantity corresponding to each pulse (for example, length per pulse, time per pulse, and weight per pulse) can be calculated.
For example, with a laser interferometer, exemplified by a Japanese Patent Application First Publication Showa 61-83911 published on Apr. 28, 1986, if a wavelength of a laser light ray is .lambda., one pulse is derived from the sensor whenever a change of .lambda./2 in displacement occurs. If the wavelength is, e.g., 0. 7034 micrometer and the accumulated number of pulses derived from the sensor is N, a final measured value of displacement V is expressed as follows: ##EQU1##
In this way, the measured value can be derived by multiplying the accumulated number of pulses N by .lambda./2.
When the displacement is increased at a constant speed .omega., an actual displacement changes as denoted by a broken line of FIG. 4 with respect to the time t. Moreover, if the time required to execute the calculation of the equation (1) is .DELTA.T, the calculated value V is derived a time .DELTA.T after the number of pulses N are latched, corresponding to a point Po at a time t.sub.o of FIG. 4. However, after the pass of time .DELTA. T, the displacement is increased by .epsilon..sub.1 (=w .DELTA. T) so that the measured value V with respect to time t is changed as denoted by a solid line. Hence, with an offset value thereof neglected, a maximum of .epsilon..sub.1 of error occurs in the measured value, as appreciated from FIG. 4.
In order to reduce the error of the measured value V, the calculation time .DELTA. T of the equation (1) may be reduced. To meet such a demand, a high speed, expensive and large scale calculating processor needs to be used. It is inconvenient for such a calculating processor to be installed in the laser interferometer.
Furthermore, when the values of effective digit numbers of the wavelength and accumulated number of pulses N become large, a limitation on shortening the calculation time arises. Additionally, the concept of driving the measured value described above cannot be used in an application requiring a real time processing field used as a feedback element of an NC (Numerical Control) machine tool.
The measured value V derived by the calculation in the way expressed in the equation (1) has, e.g. a decimal fraction, for instance a fraction of 0.2755 in a number 5.2755. However, an accuracy of a measured value V generally corresponds to a quantity of one pulse, and less importance is placed on its decimal fraction. Hence, if one pulse of .lambda./2, is i.e., 0.3517 .mu.m, the measured value V has a resolution of 1 .mu.m, with eliminating the decimal fraction. Furthermore, with the decimal fraction eliminated, a pulse train whose interval corresponds to the physical quantity (for example, 1 .mu.m) fraction is eliminated is formed to count the number of pulses in the pulse train. It should be noted that in the method for carrying out the calculation in the way of equation (1), the forming of the pulse train whose interval corresponds to the physical quantity from which the decimal fraction is eliminated generally results in high cost.